Fuel cell is a power-generating device that is high in power generation efficiency, and byproduct thermal energy can also be used effectively. Since fuel cell generates electricity by a chemical reaction, it has a power generation efficiency that is higher than that of other power-generating systems, in which electricity is generated secondarily, such as combusting fuel to make steam to rotate a turbine. The product of fuel cell is theoretically water, and it does not combust fuel. Therefore, emission of carbon dioxide is small, and it does not emit nitrogen oxides and sulfur oxides, which cause air pollution. Thus, it attracts attention as a next-generation clean energy.
In particular, a polymer electrolyte fuel cell (hereinafter abbreviated to PEFC) using a polymer solid electrolyte membrane as an electrolyte has a high output density of electric energy to be produced and is capable of low-temperature drive, that is, generating electricity at low temperatures. Therefore, it is developed for use of an automotive power source or of a home cogeneration system using a fuel cell as a power source or heat source, etc.
It is explained that fuel cell generates electricity by an opposite reaction to electrolysis of water. That is, electrolysis of water is a reaction in which electric energy is changed into chemical energy to decompose water and obtain oxygen molecules and hydrogen molecules. In contrast, fuel cell conducts a reaction to change chemical energy into electric energy. That is, in fuel cell, a fuel (hydrogen, methanol, natural gas, etc.) introduced to a hydrogen fuel electrode releases an electron by catalyst to become a hydrogen ion (proton). Since a solid electrolyte membrane for fuel cells has a property that is not permeable to electrons, the electron goes to an external circuit through an electrode, thereby allowing an electric current to flow. The proton moves in an electrolyte membrane and reacts at an air electrode as the opposite electrode with oxygen supplied and the electron returned from the external circuit, thereby producing water.
In a fuel cell, a solid electrolyte membrane for fuel cells is an important member that influences electric power generation performance. A solid electrolyte membrane for fuel cells is required to have a low cost and a high durability. In recent years, operating temperature in the electric power generation has increased to 120° C. to 150° C. for the purpose of high efficiency by using waste heat, the catalyst poisoning reduction, etc. Furthermore, it is required to omit or simplify a humidifier for the purpose of lowering the cost of a fuel cell system, that is, to have a high proton conductivity even under a low humidified condition of 20% or less humidity.
For a solid electrolyte membrane for fuel cells, a perfluorocarbon sulfonic acid series polymer is used. As solid electrolytes for fuel cells by perfluorocarbon sulfonic acid series polymers, a trade name Nafion from US Aldrich Co., a trade name Flemion from Asahi Glass Co., Ltd., a trade name Aciplex from Asahi Kasei Corporation, a trade name GORE-SELECT from Japan Gore-Tex Inc., etc. are on the market.
Sulfonic acid group in a perfluorocarbon sulfonic acid series polymer shows hydrophilicity with water and makes a cluster moiety in the polymer. It is believed to show proton conductivity by the movement of protons with water molecules among clusters. Therefore, it is necessary to have a condition that water is contained in a solid electrolyte membrane for fuel cells in order to obtain a high proton conductivity.
Perfluorocarbon sulfonic acid series polymers are superior in chemical stability, but glass transition temperature is low, and heat resistance is low. Therefore, there has been a problem that operating temperature is low as being 70° C. to 100° C. Furthermore, there are problems that proton conductivity lowers upon a low humidification and that water cannot be maintained in the solid electrolyte membrane for fuel cells, at 100° C. or higher, thereby greatly lowering proton conductivity and mechanical characteristics.
Of PEFC's, a direct-methanol fuel cell (hereinafter abbreviated to DMFC) uses methanol in place of hydrogen and directly makes this react at an electrode to generate electricity. Unlike other fuel cells that hydrogen is converted to a hydrogen ion (proton) and an electron by removing the electron from the hydrogen by catalyst on an anode side (fuel electrode), in DMFC, methanol reacts directly with water by catalyst on the anode electrode and changes to proton, electron or carbon dioxide.
In the case of using methanol as a fuel, there occurs a crossover phenomenon that a part of methanol passes from the anode side to the cathode side, thereby causing a problem of lowering of the electric potential of the air electrode besides fuel loss.
Furthermore, a perfluorocarbon sulfonic acid series electrolyte membrane has many production steps. This tends to increase the cost. Therefore, research and development is conducted on a film prepared by introducing sulfonic acid group to an aromatic hydrocarbon series polymer, for reducing the cost.
For example, Patent Publication 1 discloses an electrolyte membrane prepared by introducing sulfonic acid group to polyarylene sulfide sulfone and/or polyarylene sulfone, but it deteriorates by the release of sulfonic acid group.
Furthermore, Patent Publication 2 discloses an aromatic hydrocarbon series electrolyte membrane prepared by introducing a sulfoalkyl group to a side chain in order to suppress the release of sulfonic acid from the aromatic ring, but it has a problem of inferiority in oxidation resistance. Thus, deterioration by oxidation is problematic in aromatic hydrocarbon series electrolyte membranes. In one prepared by directly introducing sulfonic acid group or the like to an aromatic, a noticeable release of sulfonic acid tends to occur at high temperatures. Therefore, it is not preferable as a high-temperature operating membrane. Furthermore, aromatic hydrocarbon series electrolyte membranes as a whole have a problem that proton conductivity lowers extremely upon a low humidification.
Furthermore, Patent Publication 3 mentions an electrolyte membrane prepared by a graft polymerization of a specific aromatic unit having a bistrifluoromethanesulfonylmethide group with no use of a sulfonic acid group. It is disclosed that an electrolyte membrane containing a perfluorosulfonyl group has proton conductivity even under a low water content condition. It is, however, necessary to conduct a graft polymerization to a styrene series polymer. This requires much effort for the production, causing a problem in terms of cost.